“Out-of-roundness” in a turbine's stator casing directly impacts the performance of the machine due to the additional clearance required between the machine's rotating and stationary parts. As clearances are reduced, machine efficiency and output increase.
Turbine stator casings are typically comprised of a semi-cylindrical upper half and a semi-cylindrical lower half that are joined together at horizontal split-line joints that can have an effect on a casing's roundness. Attempts have been made to reduce the out-of-roundness effects associated with the use of horizontal joints by adding false flanges, which add mass at discrete locations, such as at the vertical plane of the casing. However, the added mass from the use of false flanges typically causes a thermal “lag” during the transient response of the machine.
One approach to solving this problem has been to use the symmetrical placement of bosses and/or cooling flows relative to the vertical and horizontal planes of the turbine casing. But the symmetrical placement of bosses and/or cooling flows has resulted in reduced cooling flows at the joints and flanges.
Another approach has been to add fins in the cooling passage of the casing at the circumferential locations where the flanges are located, so as to provide more surface area for improved cooling and heating. But this approach is limited when cooling flows are reduced due to symmetry planes. By increasing heat transfer in those regions where the horizontal joints and false flanges are located, “out-of-roundness” can be reduced, which, in turn, allows machine clearances to be reduced.